Degenerative disc disease results in significant pain for a large population of patients. The current standard of care is an instrumented fusion using a variety of surgical approaches. In the procedure, the intervertebral disc is removed (discectomy) and replaced with a stiff strut or cage. Many times, additional plates, rods and screws are used to stabilize the construct as it heals. The healing process has the goal of vertebral body bony fusion across the disc space. In some cases, as many as 20%, fusion does not occur. Instrumentation, such as cages and screws and rods can migrate, causing serious neurological complications. In patients who smoke, fusion success is even smaller. For those patients in whom fusion was successful, there appears to be a greater likelihood of adjacent disc failure. While this has not been conclusively proven, it is clear that the adjacent levels are subject to higher stresses due to loss of range of motion at the fused disc. Additionally, there is a loss of mobility at the fused level that varies with location in the spine.
Another segment of the population suffers from spine deformities such as scoliosis. Scoliosis is also a degenerative disease in that the spine curvature progresses and may result in cardiopulmonary distress.
In some patients treatment of degenerative disc disease with a prostheses may not be tolerated or the device may fail. In those cases the implanted device should have a strategy for conversion to fusion.
Many of the current devices used in spine disc replacements can be characterized as being generally stiff. These devices are often fabricated from metals and plastics. Some may include a softer core element. U.S. Pat. No. 6,881,228 B2 to Zdeblick teaches an artificial disc implant having for replacing the spinal disc between two vertebrae of the spine. The implant employs an upper and lower shell with a spacer sandwiched in the space there between. The upper and lower shells are composed of rigid materials and generally use ribbed protrusions at the vertebral interfaces. The spacer utilizes a hydrogel substance within a single cavity. At insertion the hydrogel substance is maintained in a dehydrated state to reduce the volume of the spacer thereby facilitating insertion into the vertebral space. Following insertion the hydrogel is rehydrated. These inflexible devices are generally difficult to insert and require insertion via an anterior insertional approach. Moreover, as a result of their rigid shape they are generally unable to adopt a shape conforming to the environment into which they have been inserted.
Due to the inherent limitations of an inflexible disc, a new generation of more flexible replacement discs has been developed. These prosthetic spinal disc replacements have flexible coverings surrounding a dehydrated hydogel core. U.S. Pat. No. 6,602,291 B1 to Ray et al. teaches a prosthetic spine disc nucleus for implantation into a nucleus cavity of a spinal disc. The prosthetic device is a constraining jacket of flexible but inelastic composition surrounding a single hydrogel core. The device is implanted in a dehydrated state with a fixed quantity of hydrogel contained within the jacket. By virtue of its dehydrated state, the size of prosthesis is reduced, thereby facilitating its insertion and enabling the device to be implanted via a posterior insertional approach. Following implantation the hydrogel core expands from a dehydrated state to a hydrated state. This prosthetic device is designed to replace only the nucleus of the disc rather than the entire disc. Furthermore, while addressing issues related to motion restoration or preservation, these devices do not address the need of subsequent conversion to a fusion mass should the biomechanical behavior of the device degrade with time.
Accordingly, what is needed in the art is a total disc replacement that restores normal biomechanical behavior while eliminating pain, restoring full function of the spine, provides a treatment for deformity, and has a strategy for conversion to fusion.